As the power needs of society increase and with the depletion of fossil fuels, there is a need for power services that provide clean efficient power. Such needs exist both for mobile applications such as the automotive industry and stationary applications as powering manufacturing facilities or commercial enterprises. To meet these needs, electrochemical fuel cells have been developed to convert the chemical energy of a fuel directly into electrical energy thereby providing a clean and efficient source of electrical power. Generally, a fuel cell includes a pair of electrodes arranged across an electrolyte, wherein the surface of one electrode is exposed to hydrogen or a hydrogen rich gaseous fuel, and the surface of the other electrode is exposed to an oxygen-containing oxidizing gas, typically air. Inside the fuel cell, hydrogen rich gas from the fuel source reacts electrochemically at a first electrode (anode) and is converted into protons and electrons by a catalyst. When converted, the protons move through an electrolyte to a second electrode (cathode) that also includes a catalyst. The catalyst induces oxygen from an air supply to combine with the hydrogen protons and electrons to form water, which is expelled from the fuel cell as vapor. The involvement of hydrogen and oxygen in the two reactions, one releasing electrons and the other consuming them, yields electrical energy across the anode and cathode by way of an external circuit, thereby generating electrical power.
Many electrochemical fuel cells employ a membrane electrode assembly (“MEA”) in which the intermediate electrolyte comprises a solid polymer electrolyte or ion-exchange membrane disposed between two porous electrically conductive electrode layers (the anode and the cathode). At the anode, the fuel (H2) is directed through a porous layer of the anode where it can be oxidized by the electrocatalyst to produce protons and electrons from the hydrogen rich fuel. The protons migrate through the polymer electrolyte membrane toward the cathode electrocatalyst to bind with the oxygen and separated electrons from the hydrogen. Once across the polymer electrolyte membrane, the oxidant (O2) enters through the porous cathode to react with the protons and electrons on the cathode electrocatalyst to form water. The electrons travel from the anode to the cathode through an external circuit, which produces an electrical current.
The basic reaction for powering a hydrogen based the fuel cell is as follows:
                              Anode          :                                          ⁢                      2            ⁢                          H              2                                      ->                              4            ⁢                          H              +                                +                      4            ⁢                                                  ⁢                          e              -                                                                        Cathode          :                                          ⁢                                    O              2                        +                          4              ⁢                              H                +                                      +                          4              ⁢                                                          ⁢                              e                -                                                    ->                  2          ⁢                      H            2                    ⁢          O                                                  Overall          :                                          ⁢                                    2              ⁢                              H                2                                      +                          O              2                                      ->                  2          ⁢                      H            2                    ⁢          O                    
A process known as reforming produces hydrogen from hydrocarbon fuels such as methanol or gasoline. Unfortunately, the stream of fuel produced by a reformer contains impurities that inhibit the desired electrochemical reaction within the fuel cell. These impurities are absorbed chemically or physically on the surface of the anode electrocatalyst and prevent H2 from bonding to active electrocatalyst sites on the anode where it can be broken down into its protons and electrons. By disrupting the anode reaction, the number of electrons, traveling from anode to cathode is reduced and the efficiency of the fuel cell is detrimentally affected. Impurities in the fuel stream that reduce the efficiency are known as electrocatalyst “poisons” and their effect on fuel cells is known as “electrocatalyst poisoning.” Electrocatalyst poisoning results in reduced fuel cell performance thereby reducing the voltage output of the fuel cell for a given current density.
Reformate fuel streams derived from hydrocarbons such as methanol (CH3OH) contain high concentrations of H2 and are well suited to fuel the electrochemical fuel cell. However, such fuels also contain electrocatalyst poisons such as carbon monoxide (CO) that exist in relatively small quantities in the fuel stream used to supply hydrogen rich gas to the fuel cell. The basic reactions for using methanol fuel to provide a hydrogen rich gas through a reformer for the fuel cell is shown as follows:
                                                        (              1              )                        ⁢                                                  ⁢                          CH              3                        ⁢            OH                    ->                                    2              ⁢                              H                2                                      +            CO                          ⁢                                                                                              (              2              )                        ⁢                                                  ⁢            CO                    +                                    H              2                        ⁢            O                          ->                              H            2                    +                      CO            2                                                                        Overall            :                                                  ⁢                                                            CH                  3                                ⁢                OH                            +                                                H                  2                                ⁢                O                                              ->                                    3              ⁢                              H                2                                      +                          CO              2                                      ⁢                                      
However, the above reactions do not practically result in the conversion of 100% of the carbon monoxide to CO2 and causes this impurity to enter the fuel cell. In fact, most reformers typically produce hydrogen gas containing up to 1% carbon monoxide. Additional steps can be taken to further reduce the carbon monoxide levels to around 10–100 ppm, but under normal operation of the reformer, there are transients that may cause the carbon monoxide levels to exceed the set points of normal operation for the reformer and the fuel cell. Even minute amounts of carbon monoxide can cause substantial degradation of the fuel cell performance. To reduce the effects of poisoning on the anode electrocatalyst by impurities like carbon monoxide created by the incomplete reaction of trace amounts of carbon monoxide from the above equation, it is possible to pre-treat the fuel supply stream prior to it entering the fuel cell. However, these pretreatment methods for fuel streams cannot effectively remove 100% of the carbon monoxide or other impurities that interfere with fuel cell efficiency. Even trace amounts of 10 ppm can result in electrocatalyst poisoning and cause a substantial reduction in fuel cell performance. Increasing the pressure of a fuel cell can reduce the ability of impurities to bond with the electrocatalyst. However, maintaining the fuel cell at a higher pressure reduces both the operational life of the fuel cell as well as the recovery rate of the fuel cell. Recovery rate is the rate at which voltage rates increase from a poisoned state to a normal state over time and generally measure in volts per minute. For example, a poisoned fuel cell with 3000 ppm CO may recover at 0.10 V/min at 101 kPa, but recovery only at 0.04 V/min at 202 kPa. Additionally, higher pressures can damage the fuel cell and MEA resulting in a shortened operational life. Therefore, maintaining a higher pressures reduce the overall efficiency and useful life of the fuel cell. It should be noted that while carbon monoxide is used in the above discussion, other impurities such as H2S, NH3, or other elements or compounds also degrade the performance of fuel cells at both the anode and cathode. Impurities can exist at the cathode that also degrade performance of the fuel cell and include air polluted with hydrocarbons such as that found in close proximate to combustion engines or fuel stations. Accordingly, the ability to reduce electrocatalyst poisoning of a fuel cell is a problem to which significant attention should be directed.
Therefore, it is an object of the present invention to manipulate the pressure of a fuel cell to reduce the ability of impurities in the fuel cell fuel stream to bind with active electrocatalyst sites.
It is another object of the present invention to manipulate the pressure of the fuel cell to reduce the effect of impurities while reducing deterioration of the membrane electrode assembly.
It is another object of the present invention to manipulate the pressure of a fuel cell to enhance voltage recovery rates to provide a more uniform power output of the fuel cell.